1. Field of the Invention
The present invention relates to lithium secondary batteries, and more particularly to improvements of the positive electrode active material in a battery system using a negative electrode active material containing silicon.
2. Description of Related Art
Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, lithium-ion batteries, which perform charge and discharge by transferring lithium ions between the positive and negative electrodes, have been widely used as a driving power source for such mobile information terminal devices.
The mobile information terminal devices tend to power consumption as the functions of the devices, such as moving picture playing functions and gaming functions. It is strongly desired that the lithium-ion batteries that are the drive power source for the devices have further higher capacities and higher performance in order to achieve longer battery life and improved output power. In view of these circumstances, attempts have been made to use an aluminum alloy, a silicon alloy and a tin alloy, which can absorb a large amount of lithium per unit volume, as the negative electrode active material so that the discharge capacity of the lithium secondary battery can be increased.
In the lithium secondary battery that employs the just-described negative electrode active material, however, the negative electrode active material pulverizes or peels off from the negative electrode current collector because the negative electrode active material undergoes a great volumetric change during absorption and release of lithium. This causes breakage of the current collection structure in the negative electrode. As a consequence, electron conductivity within the negative electrode degrades, and the charge-discharge cycle performance becomes poor.
In view of the problem, Patent Document 1 discloses a negative electrode that achieves good cycle performance. This negative electrode is formed by sintering a negative electrode active material layer containing a polyimide binder and a negative electrode active material composed of a material containing silicon under a non-oxidizing atmosphere, in order to obtain good current collection capability within the electrode.